Our invention comprises improvements in a continuously variable transmission design of the kind disclosed in U.S. Pat. No. 4,767,384. Other continuously variable transmission designs capable of embodying the improvements of my invention are shown in U.S. Pat. Nos. 4,827,805 and 4,836,049, as well as copending patent application Ser. No. 29,019, filed Mar. 8, 1993 by Thomas Tibbles, one of the inventors of the present invention, and by Pramod Jain. The copending application and the three U.S. patents identified above are assigned to the assignee of our present invention.
It is common practice in the design of a continuously variable transmission (CVT) for automotive vehicles to match the ratio changing characteristics of the infinitely variable transmission with the speed/torque characteristics of a throttle-controlled internal combustion engine for the vehicle so that the powertrain package comprising the engine and the transmission will operate, under normal driving conditions, so that the engine and the transmission will achieve minimum brake specific fuel consumption for each engine throttle setting. This requires a continuous change in the ratio of the transmission as the throttle position and the engine speed change to meet the driving demands of the driveline. The adjustment in the transmission ratio is achieved by using primary driving and secondary driven sheave assemblies having relatively axially adjustable friction drive portions with juxtaposed cone surfaces. As the cone surfaces of each sheave assembly are adjusted, one with respect to the other, the position of a drive belt or chain located between the friction surfaces will change in a radial direction so that the overall ratio of the transmission will vary from a minimum underdrive ratio to a maximum overdrive ratio.
Adjustment of the friction drive portions of each sheave assembly of the CVT transmission disclosed in the copending application identified above is achieved by a primary fluid pressure operated servo comprising a piston located within a pressure cylinder. By varying the effective pressure on the primary sheave assembly servo, the ratio of the drive can be changed. The clamping pressure on each sheave assembly can be controlled by maintaining an effective clamping pressure on the secondary sheave assembly by a secondary fluid pressure operated servo.
The control pressure that is distributed to the working chambers of the primary servo and the secondary servo of the CVT transmission of the copending application identified above is established by a control system that includes a pump driven by the engine. A regulator valve in the control system for the transmission achieves the desired working pressure in the sheave assemblies so that slippage of the belt or chain will not occur. The control system also includes a ratio control valve arrangement for controlling pressure distributed to the primary sheave assembly to effect adjustment of the spacing between the sheave portions of the primary sheave assembly, thereby changing the overall ratio of the transmission.